Generally speaking, in the case where an automobile body constitutes the object being coated, the formation of a multilayer coated film is carried out by a method in which an electrodeposited coat is first formed on the object being coated and then heating and hardening carried out, after which a multilayer coated film comprising an intermediate coat, a basecoat and a clearcoat are formed. Currently, waterborne coating materials are being employed as the intermediate and basecoat coating materials in order to reduce the use of volatile organic compounds (VOC).
In recent years, from the point of view of saving energy, a method for forming the multilayer coated film by means of a so-called 3-coat 1-bake (3C1B) system has begun to be adopted where, instead of applying the intermediate-coat coating material onto the electrodeposited coat (the electrocoat) to form the intermediate coat and then subjecting this intermediate coat to heating and hardening, the basecoat and the clearcoat are formed on the intermediate coat after this has undergone low-temperature drying (flash-off), and then these three coats are subjected to simultaneous heating and hardening.
With regard to the method of forming a multilayer coated film by the 3C1B system, in Patent Document 1 there is disclosed a method for forming a multilayer coated film by the 3C1B system using a first waterborne coating material, a second waterborne coating material and a clearcoat coating material where, in said method for forming a multilayer coated film, by including in the second waterborne coating material an emulsion resin having a core/shell structure which is obtained by the emulsion polymerization of a specified vinyl monomer mixture, and also including a urethane emulsion, etc., there is obtained a multilayer coated film which is excellent in its external appearance and water resistance.
Furthermore, in Patent Document 2 there is disclosed a method for forming a multilayer coated film by the 3C1B system using a waterborne first colored coating material, a waterborne second colored coating material and a clearcoat coating material where, in said method, by including in the waterborne first colored coating material a specified hydroxyl group-containing polyester resin, and a water-dispersible acrylic resin having a core/shell structure obtained by the polymerization of specified monomer components, etc., there is formed a multilayer coated film which is outstanding in terms of smoothness and image clarity.
However, in the methods for forming a multilayer coated film by the 3C1B system described in aforesaid Patent Documents 1 and 2, when flash-off is omitted following the formation of the intermediate coat in order to achieve still greater energy savings, layer mixing occurs between the intermediate coat and the basecoat, and a satisfactory coated film external appearance is not obtained.